GIS HV Cable Termination -Protection of GIS-Cable terminations against HF TEV
Transient enclosure voltage (TEV) in gas-insulated substations (GISs) is due to transients which appear during switching operations such as disconnector opening and closing inside a GIS substation.
The HF transient overvoltages are generally confined to the inside of the screening provided by GIS enclosures.
The GIS enclosures also include discontinuities. The discontinuities may allow the high frequency effects transferred to the exterior of the GIS enclosures as it is the case for HV cable terminations.
The GIS cable terminations should be mounted with sheath sectionalizing insulators between the components electrically connected to the GIS enclosure and the cable sheath.
Methods to protect the system from the high frequency transient overvoltages at the discontinuities between the GIS enclosures and cable terminations:
1) Install a metallic earth connection between the GIS enclosure flange and cable sheath. It avoids overvoltage between the GIS enclosure and the cable sheath but causes permanent circulation of current in a closed loop formed by the multiple connections to ground.
The SVLs need to be mounted close to the gap to be protected and connected by short low-impedance leads.
2) Install a non-linear resistor/bypass Sheath Voltage Limiters (SVLs) across the sheath sectionalizing insulator to limit the overvoltage under transient conditions between the components electrically connected to the GIS enclosure and the cable sheath.
In the case of single-point bonding connection where SVLs are installed on the GIS side, the sheath temporary overvoltage that appears between cable sheath and earth when a phase-to-ground fault occurs is seen by both limiters: the sheath voltage limiter which protects the cable sheath, and the bypass sheath voltage limiter located between the cable sheath and the GIS enclosure. Therefore, the bypass SVL’s rated voltage should be equal or higher than the sheath voltage limiter rated voltage.
Source: CIGRE
